add several TODO comments

self-note: check this one after you come back!

src/engine/engine.cpp

@@ -414,6 +414,7 @@ void DivEngine::runExportThread() {
   }
 }
 
+// TODO: properly export ExtCh songs in per-channel mode
 bool DivEngine::saveAudio(const char* path, int loops, DivAudioExportModes mode) {
   exportPath=path;
   exportMode=mode;

src/engine/waveSynth.cpp

@@ -2,6 +2,10 @@
 #include "engine.h"
 #include "instrument.h"
 
+// TODO:
+// - add missing license header
+// - implement waveSynth on Amiga and N163
+
 bool DivWaveSynth::activeChanged() {
   if (activeChangedB) {
     activeChangedB=false;